Method of producing impregnated wooden products

ABSTRACT

The present invention relates to a method of producing an impregnated wooden product from a wooden article. The article is pressed isostatically with a pressure exceeding 1,000 bars in a first method step. The wooden article is then placed in a bath containing a liquid-impregnating agent in a second method step and allowed to swell so that at least a part of the impregnating agent is absorbed by the article. The method may optionally be supplemented with a third method step in which the article is pressed isostatically with a pressure exceeding 1,000 bars, so as to form a hard element. The third method step may alternatively be applied to a conventionally pressure-impregnated article.

This application is the national phase of international applicationPCT/SE96/00126 filed Feb. 2, 1996 which designated the U.S.

The present invention relates to a method of producing impregnatedwooden products from wooden articles.

Various impregnating methods are known for treating wooden articles withthe intention of preventing the occurrence of wood mould and wood rot.Such methods may involve placing the articles in a chamber, evacuatingthe chamber, introducing an impregnating agent thereinto, and thenre-pressurizing the chamber. The impregnating liquid enters the woodcells to a greater or a lesser extent.

It is also known to produce hard elements, for instance floor boarding,by compressing different types of wooden products in conventionalpresses. The pressures applied in this regard do not normally exceed 500bars, even though much higher pressures have been proposed. The use ofthese much higher pressures has not found practical use in the presentcontext however, due to the fact that in conventional presses these highpressures can only be achieved in the case of very limited surfaceareas.

Application of this latter method normally results in considerabledeformation of the material during compression. This deformation is notonly caused in the direction in which the pressing forces act, but alsotransversely to this direction. This is because the material is pressedbetween an upper and a lower press plate which enables the woodenarticles to expand freely in a lateral direction. When counter-pressuredevices are used to impede this lateral expansion of the material, thepresses become complicated and their use is limited to specificdimensions; see for instance Swedish Publication SE 601162 in thisregard.

Patent abstract of Japan, vol. 16, No. 153, m-1235, abstract ofJP-A-4-7101, discloses a method where wooden articles are compressed ina conventional pressing apparatus in a first step. The article is thenimmersed in a treating liquid in order to impregnate the article.Finally, the article is dried by treatment with heat.

Swedish Patent Application 9303821-4 describes a method of producinghard elements from a wooden article by applying very high pressures,i.e. pressures higher than 1,000 bars, and an isostatic pressingprocess, i.e. a process in which a uniform pressure is applied over thewhole of the outer surface of the article. These high pressures can beapplied to large surface areas by means of a so-called Quintuspress.

The present invention relates to a further development of the methoddescribed in the aforesaid Swedish and Japanese patent applications, andthe object of the invention is to provide an impregnating method whichwill produce fire-retardant and/or rot-resistant and/or shape-durableelements and hard, impregnated elements respectively.

This article is achieved with a method having the characteristicfeatures set forth in the following claim.

In the method according to the present invention, a wooden article ispressed isostatically in a first step at a pressure in excess of 1,000bars, either in a so-called Quintuspress or by means of some otherappropriate pressing method. In this regard, the extent to which thearticle is compressed will depend on its dry solids content, the fibredirection, its hardness and other properties. However, one of thecross-sectional dimensions of the article will normally be reduced bybetween 20-50% in the pressing operation, wherewith the wood cells arepressed together. The wooden article is then relieved of load.

The article is placed in a liquid bath containing impregnating agent ina second step, wherein the article successively swells as a result ofliquid penetrating into and being absorbed by the wood cells. The liquidpenetrates into the cells so that the article is completely impregnatedthroughout. Expansion of the article is contingent on the materialproperties thereof and also on the time during which the article remainsin the bath, among other things. Normally, a marked expansion takesplace within the course of some hours, and the material returns to itsoriginal form within this time period in certain cases. Suitableimpregnating agents are fire-retardant liquids, rot-retardant liquids orglue. However, in order for the liquid to penetrate into the cells, itis necessary for the liquid to have a high wood penetration capacity.

Tests have been carried out with pine test pieces having a thickness of24 mm, which was reduced to 12 mm by the isostatic pressing operation,whereby the density of the test pieces increased to immediately below 1kg/dm³. When placed in a bath containing a fire-retardant agent, thetest pieces expanded to their original size within the space of onehour. The test pieces were then dried and divided into smaller parts andattempts were made to ignite parts which had been located centrally inthe article and parts which had been located on the sides thereof. Itwas, however, impossible to ignite any of these parts, which indicatedthat the fire-retardant liquid had penetrated to the central parts ofthe article.

In the third step of the aforesaid method, the article is againcompressed isostatically at a pressure exceeding 1,000 bars, therewithobtaining a hard element which is fire-resistant, rot-resistant and alsoshape-durable when glue is used.

We claim:
 1. A method of producing an impregnated wooden product from awooden article, comprisinga) pressing the wooden article tosubstantially compact the wood; b) after step a), immersing thecompressed wooden article in a bath of liquid impregnating agent wherebythe immersed article is permitted to swell so that at least a part ofthe impregnating agent is absorbed by the article; and c) after step b),pressing the wooden article a second time, characterized in that thewooden article is compressed by an isostatic pressure exceeding 1,000bars in steps a) and c); and where the impregnating agent is afire-retardant agent, a rot-preventing agent or a glue.